Since the early 1970's, microwave radio has been gaining importance as a transmission medium for digital communications. The telecommunication administrations of most countries have digitized transmission and switching facilities and have developed integrated digital networks. The present transmission network consists of a mix of many different communication channels such as fiber optic, cable, coaxial, satellite, line-of-sight radio systems and so on. Digital radios have achieved extreme success due to the fact that their spectral efficiencies are compatible with those of their analog counterparts and they allow coexistence with other transmission systems which do not lead to overall performance degradations. To assure that these criteria are adapted uniformly on a world-wide basis, regulatory bodies such as the CCIR, CCITT, and FCC have approved and provided a number of reports and recommendations which enable consistent transmission and reception and enable various manufacturers to provide communication receivers and transmitters which are totally compatible with one another. The reports and other recommendations from such agencies are widely known and are widely utilized today. In general for the operation of digital radios, reference is made to a text entitled Microwave Digital Radio, edited by Larry J. Greenstein and Mansoor Shafi and published by the IEEE, New York, 1988.
It is accurate to say that various protocols for the transmission and receiving in digital radios have been promulgated by the above-noted agencies. An extremely popular protocol which is widely known, is designated as the X.21 protocol. The X.21 protocol provides full duplex communications across point-to-point circuits such as lead circuits using the Higher Data Level Communication (HDLC) protocol. This protocol uses the control (C) and indicate (I) leads as circuit interface signals to indicate the on/off state of directly connected Data Communication Equipment (DCE) and Data Terminal Equipment (DTE).
The Higher Data Level Communication (HDLC) protocol has been is use for many years as a serial communication protocol. The protocol provides full duplex link level communications between data terminal equipment interconnected by wire lines or circuit switched networks.
The protocol uses a continuous synchronous data stream consisting of abort, flag, and zero inserted information characters. When the communication line is not transmitting information characters it is transmitting flag characters. Zero inserted information characters are concatenated together to form data frames. Data frames are bounded by opening and closing flag characters.
The characteristics of the X.21 HDLC protocol signals are very well known. The X.21 HDLC requires that the circuit be connected end to end, both C and I lead asserted and a continuous exchange of flags before advancing from the ready state to the data transfer state. Once in the data transfer state, either end may transmit data simultaneously.
The X.21 and HDLC protocols do not provide the ability to interface with half duplex radios. A half duplex radio, as one can ascertain, is a device that can only transmit in one mode and receive in another mode. The half duplex radio can not transmit and receive simultaneously. As indicated, there is no mechanism defined in the X.21 protocol to indicate push-to-talk transmit action with the radio and there is no mechanism to determine when the radio is busy receiving. The current X.21 and HDLC protocols are defined as data only protocols. The X.21 and HDLC protocols do not support mixed voice and data operation across half duplex radios. The X.21 and HDLC protocols do not provide mechanisms to share a communication media between human voice user and a data terminal. The X.21 and HDLC protocols do not allocate radio resources to voice or data use because there is no capability provided within the protocol to determine when the communications media is being used for voice communications.
It is therefore an object of the present invention to modify the X.21 and HDLC protocols so they are adaptable to operation with half duplex radios.
It is an further object to modify the above-noted protocols so they are compatible with operation with half duplex radios while maintaining the ability of full duplex radios to operate with the protocol in a totally unimpeded manner.